I. Field of the Invention
The present invention relates to a and, particularly, but not exclusively, to a fuse arrangement having application in high voltage current carrying circuits, such as found in country-wide electrical distribution networks.
Fuses are provided in electrical distribution networks to prevent damage from fault currents such as may be caused by overload and short-circuit conditions. The function of a fuse is to prevent large amounts of electrical energy flowing in the circuit in short periods of time, thereby avoiding damage to circuit components and devices connected to the circuit. To perform this energy limiting task, some fuses operate to, firstly, limit the current flowing in the current carrying circuit following the occurrence of a fault current and, secondly, to break the current carrying circuit to prevent further current flow.
Fuse operation is a complex process and breaking of a circuit takes a finite amount of time during which current will still be flowing. Hence the need, particularly in circuits generating high currents, for fuses which also perform a current limiting operation. Fuse operation generally involves fusing of the fuse element, formation of a fuse arc (at this time current is still flowing in the circuit) and cut-off of the fuse arc (at this stage current is "broken").
II. Description of Related Art
Sand fuses, consisting of a long fuse element surrounded by sand in a tube, are fuses which operate to cut-off current flow very rapidly following the occurrence of a fault current. They are said to have high "breaking capacity" and can cope with very large values of fault current (12 KA or more). The sand operates to "quench" the fuse arc, usually before the current wave-form has reached its peak (in an a.c. system). Sand fuses are said in the art to have a "current limiting" operation, because they operate to cut the current off very rapidly, together with a current breaking operation. They protect the circuit from damaging quantities of electrical energy by virtue of the rapid cut-off of current.
In another arrangement the fuse is mounted within metal or insulation enclosed switch gear, indoors or in pad mounted substations. In these applications a separate load break switch is provided to permit complete disconnection of all fuses in the event that one operates on fault current. The fuse usually incorporates a mechanical triggering device designed to operate the load break switch.
A problem with sand fuses is that their operation on occurrence of fault currents of relatively low value is limited. At a relatively low value current, the fuse element "burns back" and the arc is not quenched at all, resulting in a large amount of energy being dissipated in the sand fuse (possibly causing explosion).
In some current carrying circuits, such as in electrical distribution networks, for example, fault currents may occur which vary widely in their value. An ideal fuse in such circuits must be able to deal with fault currents of very large value (12 KA or more) as well as fault currents of relatively low value. Such fuses must have a wide "operating range".
It is possible to design sand fuses with a wide range, but this necessitates complicated and expensive process engineering of the fuse element and the provision of a back-up fuse to deal with the "minimum breaking-capacity" current (lowest value fault current at which the fuse must break the current carrying circuit). Such complex fuses are one-shot only and their replacement is expensive. That is, there is a relatively high cost of "fault clearing" for these fuses.
An expulsion fuse is a simple type of fuse merely comprising a fuse element which may extend in a tube, with no sand. The expulsion fuse element is generally relatively short compared with the elements used in sand fuses. Traditional expulsion fuses operate in a different way to sand fuses. They have no "current limiting" function, merely operating to break the current in the current carrying circuit. The fuse arc in an expulsion fuse is not quenched until current flow approaches naturally to zero. This means that up to half an a.c. wave form of current may flow before the circuit is broken. Such a long time of operation increases the energy dissipated in the fuse. To restrict this, moderate breaking currents only are permitted. That is why expulsion fuses only have medium rated breaking capacities. They are not useful with very large currents, such as some fault currents which can be expected in electrical distribution networks because the amount of electrical energy which may flow in the current carrying circuit may be sufficient to cause the fuse carrier (the housing mounting the fuse element) to explode, causing damage to the current carrying circuit in the immediate vicinity of the fuse. Even the medium rated expulsion fuses will operate with a very loud noise as the energy of the fuse arc is dissipated within them. Expulsion fuses, however, can operate at relatively low current levels. They are therefore suitable for low to medium range applications. In addition, they are relatively cheap and the cost of fault clearing with these types of fuses is low.
One known expulsion fuse is mounted in a carrier arranged to "drop out" mechanically from the circuit once the fuse element has broken. These are known as "drop out" fuses.